The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Two methods of improving fuel economy in passenger vehicles that have been employed in the art include reducing the weight of the vehicle, and developing high-performance engines. To increase engine efficiency, the maximum operating temperature of engine components has increased from approximately 170° C. in earlier engines to peak temperatures well above 200° C. in recent engines. The increase in the operational temperatures requires a material with improved properties in terms of tensile, creep and fatigue strength. Cast aluminum alloys based on the Al—Si eutectic system with Cu and Mg additions, such as AA319, AA356, and AS7GU, have been widely used in automotive engine blocks and heads due to their low density, high thermal conductivity, good castability, and excellent low-temperature strength.
These cast aluminum alloys achieve their strength primarily from coherent or semi-coherent precipitates that form during post-solidification heat treatment, for example θ′-Al2Cu, Q′-Al5Cu2Mg8Si6 and β′-Mg2Si precipitates. These small precipitates are generally metastable rather than being in an equilibrium phase. As a result, the above-mentioned aluminum alloys lose their strength at elevated temperature because these metastable strengthening precipitates dissolve into the Al matrix or coarsen to equilibrium phases that do not provide the same level of strengthening. Experimental data show that the yield strength and ultimate tensile strength of AA319 alloy with a T7 heat treatment drops dramatically when exposed to temperatures between 170° C. and 200° C. In addition, the alloy endurance limit decreases from 88±6 MPa at room temperature to 62±8 MPa at 120° C.
A common strategy to improve the elevated-temperature performance of cast aluminum alloys is to modify the alloys with the addition of transition metals (TM). These TMs form thermally stable precipitates L12-Al3TM, which are resistant to coarsening at high temperatures. However, for the vast majority of these Al-TM alloys, TMs are added to a dilute aluminum alloy, leading to very poor room-temperature performance, since the solubility of TMs in the Al matrix is so small that the volume fraction and density of these precipitates are insufficient to provide significant strengthening. For example, the maximum solubility of Ti, V, and Zr in Al is 1 wt. %, 0.6 wt. %, and 0.25 wt. %, respectively, much smaller than that of commonly used strengthening elements such as Cu (4.7 wt. %) and Mg (14.9 wt. %).
Improving high-cycle fatigue and performance at elevated temperatures for cast aluminum alloys having select TMs, especially in motor vehicle engine applications, is addressed by the present disclosure.